Bistable magnetic thin film rod having a conductive overcoating



E. M. WILLER 3,516,075

BISTABLE MAGNETIC THIN FILM ROD HAVING A CONDUCTIVE OVERCOATING June 2, 1970 2 Sheets-Shee'h 1 Filed 00%.. 4, 1965 VII '14;

h J j a mulling Ilium" INVENTOR ERNEST M. WILLER 54 Hl ATTORNEYS E. M. WILLER June 2, 1970 BISTABLE MAGNETIC THIN FILM ROD HAVING A CONDUCTIVE OVERCOAIING 2 Sheets-Sheet 2 l I wn'HouT cow OVERPLATE 30o WRITE PULSE CURRENT (MILLIAMPERES) w. I A m 5 m m w F D L o O G 0 H 2 W W 5 6 9 1a A w w 0 O 0 0 1 m w M w .W m 0 l m N350, wmzoumwm M ATTORNEYS E m I T l m a W M l T 5 J E w R M. E

Y m D 8 v m 0 E D M L T m w W T TE I V J M 1 F IAN-HI fi fi l United States Patent 3,516,075 BISTABLE MAGNETIC THIN FILM ROD HAVING A CONDUCTIVE ()VERCOATING Ernest M. Willer, Westminster, Calif., assignor to The National Cash Register Company, Dayton, Ohio, a

corporation of Maryland Filed Oct. 4, 1965, Ser. No. 492,496 Int. Cl. Gllc 11/04, 11/14 US. Cl. 340-174 12 Claims ABSTRACT OF THE DISCLOSURE A bistable magnetic rod memory device including a thin film of bistable magnetic material uniformly deposited on a conductive substrate and a thin non-magnetic conductive overcoating uniformly deposited over the magnetic material. A helical solenoid winding having an insulating coating thereon is provided over the conductive overcoating along the rod memory. The conductive overcoating, having a thickness of less than 5,000 angstroms, permits a rapid and simple connection to be made between the substrate and the solenoid winding by the simple expedient of dip soldering. The conductive overcoating additionally provides an improvement in the square-loop hysteresis characteristics of the magnetic film.

The present invention relates generally to memory devices for use in digital computer systems, and more particularly to improvements in bistable magnetic rod memory devices of the type illustrated, for example, in Pat. No. 3,134,965, issued May 26, 1964 and the commonly assigned copending patent applications Ser. No. 795,934, filed Feb. 27, 1959 and now US. Pat. No. 3,228,012, Ser. No. 268,145, filed Mar. 26, 1963 and now US. Pat. No. 3,341,829, and Ser. No. 373,980, filed June 10, 1964 now Pat. No. 3,418,644.

A particularly desirable form of rod memory element is that disclosed in the aforementioned patent applications Ser. No. 268,145 and ,Ser. No. 373,980 in which a continuous solenoidal winding is provided along the length of a rod and is soldered at one end of the rod to the inner conductive substrate so as to thereby permit the conductive substrate to serve as a current return path without having to provide an additional return wire, as would otherwise be required. As disclosed in patent application Ser. No. 268,145, the use of the inner substrate for a return path in place of a separate return wire provides the further advantages of reducing the magnetic switching field required and of cancelling the circular or transverse field produced by current flowing in the solenoidal winding.

However, although the use of the inner substrate as a return path provides the above-mentioned important advantages, it has been found most difiicult in practice to achieve a satisfactory electrical and mechanical connection between the substrate at the end of the magnetic rod and the solenoidal wire. In accordance with the present invention, this problem is solved in a most advantageous manner which also unexpectedly results in a decided improvement in the square-loop hysteresis characteristics of the magnetic thin film electrodeposited on the rod.

It is thus an object of this invention to provide an improved magnetic rod construction which permits rapid and simple electrical and mechanical connection between a solenoidal winding on the rod and the inner conductive substrate.

A further object of this invention is to provide a mag netic rod construction exhibiting improved square-loop hysteresis characteristics.

3,516,075 Patented June 2, 1970 The above objects are accomplished in accordance with the present invention by the provision of a very thin nonmagnetic conductive overcoating of preferably gold or silver on the magnetic rod which, contrary to what would ordinarily be expected, does not significantly degrade the switching output signal magnitude, if made thin enough (at least less than the thickness of the magnetic film), and yet, permits rapid and convenient electrical and mechanical soldering to the inner conductive substrate, and also, most surprisingly, achieves the further advantage of significantly improving the squareness of the magnetic thin film hysteresis characteristic. A thin gold overcoating, in particular, has been found to give a very significant and unexpected improvement in square-loop hysteresis characteristics, while also providing for good solderability.

It is important to note with respect to the present invention that, although it is well known to provide a conductive overcoating to aid in soldering, it is also well known that such a conductive overcoating on a magnetic thin film rod is, in effect, a shorted turn which would ordinarily be expected to seriously degrade the output signal obtainable because of eddy current effects, as well as presenting the problem of introducing unpredictable eifects into the hysteresis characteristics of the magnetic thin film. In fact, initial attempts at providing a conductive overcoating resulted in the expected serious degradation in output signal magnitude as well as degradation of the hysteresis characteristics of the magnetic thin film. It was only when the thickness of the non-magnetic conductive overcoating was reduced way below what is conventional for a soldering overcoating, to less than 5,000 angstroms and preferably less than the thickness of the already very thin magnetic film, that the output signal magnitude was essentially retained, and the added and unexpected bonus of an improved square-loop characteristic was achieved.

The specific nature of the invention as well as other objects, uses and advantages thereof will become apparent from the following description of the invention taken in conjunction with the accompanying drawings in which:

FIG. 1 is a pictorial schematic view of a solenoid matrix and an illustrative rod in accordance with the invention.

FIG. 2 is an enlarged pictorial view illustrating the construction and arrangement of a typical rod in accordance with the invention.

FIGS. 3 and 4 are graphs comparing the hysteresis characteristic of a rod with and without gold overplating.

Referring initially to FIG. 1, illustrated therein is an exemplary solenoid matrix in which the rod devices of the present invention (only one of which is shown in FIG. 1) may advantageously be inserted to provide the bistable elements therefor. The exemplary solenoid matrix in FIG. 1 may typically comprise a plurality of forty stacked planes or plates P P each plane containing solenoid windings (for Example 10) arranged in series-connected rows, with respectively located windings in different planes aligned so as to provide a continuous bore (for Example 10a) therethrough.

Referring now to FIG. 2, illustrated therein is an enlarged view of a rod 15 and its associated helical solenoidal winding in accordance with the present invention, one of each being adapted for insertion into a respective bore 10a in the matrix of FIG. 1. The rod 15 comprises an inner conductive substrate 15a which may typically be a six inch long beryllium copper rod of about 0.010 inch in diameter, but preferably less than 0.050 inch, on which is electrodeposited a thin bistable magnetic thin film 15b which may typically be a film of 97% iron and 3% nickel with a thickness of 10,000 angstroms or less so as to exhibit essentially single domain magnetic switching properties. The bistable magnetic thin film 1511 may also be a bilayer film of the type disclosed in the commonly assigned copending patent application Ser. No. 77,451, filed Dec. 21, 1960, and now US. Pat. No. 3,213,431namely, a first adherent iron-nickel layer electrodeposited on the beryllium copper rod 12a and composed of from approximately 30% to 90% nickel and from approximately 70% to iron, and a second adherent iron-nickel layer electrodeposited on the first layer and composed of from approximately 93% to 99% iron and from approximately 7% to 1% nickel, the composite thickness typically ranging from approximately 2,000 to 5,000 angstroms.

In accordance with the present invention, the magnetic thin film 15b in FIG. 2 is overcoated with a very thin non-magnetic conductive layer 16. A 100 to 2,000 angstroms electroplated layer of gold or silver has been found to give excellent results with regard to both hysteresis characteristics and output signal magnitude, as well as providing easy solderability to the rod. In any event, it is critical that the thickness of the nonmagnetic conductive overcoating not exceed 5,000 angstroms, and preferably should be less than the thickness of the magnetic film. If desired, a thin protective coating (not shown) of, for example, urethane resin, may be applied over the non-magnetic conductive overcoating 16 to prevent any possibility of atmospheric corrosion.

As shown in FIG. 2, a helical solenoid winding 18 having an insulating coating thereon is provided over the silver or gold overcoating 16 (and protective coating if present), and preferably is formed using an insulated fiat wire 18a helically wound from one end of the rod to the other. This helical winding 18 is electrically and mechanically connected to the substrate 15a at the back end of the rod 15 (as seen in FIG. 2), via the conductive overcoating 16, by the simple expedient of dip soldering, as indicated by the resulting dip soldered connection at the back end of the rod 15. The temperature of the dip solder bath is high enough to remove the insulated coating on the portion of the helical winding 18 which receives the solder. Even though the conductive overcoating 16 is very thin, it nevertheless permits this simple dip soldering operation to form a reliable electrical and mechanical connection between the solenoidal winding 18 and the conductive substrate 15a at the back end of the rod 15.

Still referring to FIG. 2, it will be noted that the wire 18a at the front of the rod is not soldered to the rod, since it is to pass to the helical solenoidal winding of another rod (not shown) for connection in dependence on the particular memory organization employed. In turn, the rod 15 in FIG. 2 receives a wire 18a from the helical solenoidal winding of another rod (also not shown) which, as illustrated in FIG. 2, is wrapped around the conductive overcoating 16 at the front end of the rod 15 and soldered thereto to form a soldered connection 35 which provides a mechanical and electrical connection between the wire 18a and the substrate 15a at the front end of the rod 15. This soldered connection 35 is ordinarily made after the rods are inserted and secured in their respective bores in the solenoid matrix of FIG. 1.

With regard to memory organization, it will be understood that the solenoid matrix shown in FIG. 1, using rods as illustrated in FIG. 2, may be organized, for example, as disclosed in the aforementioned patent application Ser. No. 373,980, filed June 10, 1964, in which the rows of solenoid windings 10 in each array in FIG. I serve as word windings, and the helical solenoid windi'ng on each rod (as illustrated by 18 in FIG. 2) serves both as a sense winding during reading, and as a digit winding during writing. For such an organization, current flow through the solenoidal winding 18 and the rod substrate 15a is as typically illustrated by the arrows I in FIG. 2. More specificially, the arrows I indicate current flowing to the helical solenoidal winding 18 from another rod (not shown) via wire 18a, then flowing through the helical solenoid winding 18 to the back end of the rod and to the substrate 15:: via the soldered connection 25 at the back end of the rod, and then returning to the front end of the rod via the inner conductive substrate 15a from where the current then flows via the soldered connection at the front end of the rod to the helical solenoidal winding of another rod (not shown).

Now turning to FIGS. 3 and 4, illustrated therein are graphs comparing the hysteresis characteristic of a rod with a 950 angstrom gold overplating on a 97% iron-3% nickel 4,000 angstrom magnetic thin film with a like rod without the gold overplating. It is quite apparent from FIGS. 3 and 4 that a significant improvement in the squareness of the hysteresis characteristic is achieved by the gold overplated rod, the result of which is to permit use of a memory mode with only a 2 to 1 selection ratio instead of a 3 to 1 selection ratio as was previously required.

While the foregoing disclosure has been primarily concerned with a particular illustrative embodiment, it is to be understood that variations and modifications in construction and arrangement may be made without departing from the scope of this invention. The present invention, therefore, is to be considered as including all modifications and variations encompassed within the scope of the appended claims.

What is claimed is:

1. A bistable magnetic device comprising: a conductive rod substrate, a thin film of bistable magnetic material uniformly deposited on said substrate, said thin film having a thickness less than 10,000 angstroms so as to exhibit essentially single domain switching characteristics, a thin non-magnetic conductive overcoating uniformly deposited over said magnetic material and having a thickness no greater than that of said magnetic thin film or 5,000 angstroms, whichever is less, a helical solenoid wound along said rod over said conductive overcoating, and means mechanically and electrically connecting one end of said solenoid to said substrate at one end of said rod via said conductive overcoating.

2'. A bistable magnetic device comprising: a conductive rod substrate, a thin film of bistable magnetic material uniformly deposited on said substrate, said thin film having a thickness less than 10,000 angstroms so as to exhibit essentially single domain switching characteristics, a thinnon-magnetic conductive overcoating uniformly deposited over said magnetic material and having a thickness no greater than that of said magnetic thin film or 5,000 angstroms, whichever is less, a helical solenoid wound along the length of said rod over said conductive overcoating, and a soldered connection provided at one end of said rod to electrically connect one end of said solenoid to said substrate at one end of said rod via said conductive overcoating.

3. A bistable magnetic device comprising: a conductive rod substrate, a thin film of bistable magnetic material uniformly deposited on said substrate, said thin film having a thickness less than 10,000 angstroms so as to exhibit essentially single domain switching characteristics, a thin non-magnetic conductive overcoating uniformly deposited over said magnetic material and having a thickness no greater than that of said magnetic thin film or 5,000 angstroms, whichever is less, and a helical solenoid wound along said rod over said conductive overcoating, a soldered connection provided at one end of said rod to electrically connect one end of said solenoid to said substrate at one end of said rod, a soldered connection provided at the other end of said rod to electrically connect to said substrate at said other end of said rod a lead wire from the solenoid of another rod.

4. The invention in accordance with claim 1, wherein said conductive overcoating is gold or silver.

5. The invention in accordance with claim 1, wherein said conductive rod substrate is beryllium copper.

6. The invention in accordance with claim 1, wherein said conductive rod substrate has a diameter less than 0.1 inch.

7. The invention in accordance with claim 2, wherein said thin film magnetic material comprises approximately 93% to 99% iron and approximately 7% to 1% nickel.

8. The invention in accordance with claim 2, wherein said thin film magnetic material comprises a first ironnickel layer of approximately 30% to 90% nickel and approximately 70% to 10% iron and a second ironnickel layer composed of approximately 93% to 99% iron approximately 7% to 1% nickel.

9. A bistable magnetic device comprising: a conductive rod substrate having a diameter of less than than 0.1 inch, a thin film of bistable magnetic material uniformly deposited on said substrate, said thin film having a thickness less than 10,000 angstroms so as to exhibit essentiallyl single domain switching characteristics, a thin coating of gold deposited over said magnetic material and having a thickness from approximately 100 to 2,000 angstroms, a helical solenoid Wound along said rod and over said gold overcoating, and a soldered connection provided at one end of said rod for electrically connecting one end of said solenoid to said substrate at said one end of said rod.

10. In a magnetic rod memory, a plurality of stacked substantially similar plates, each plate containing an array of solenoids arranged in rows with respectively located solenoids in different rows aligned so as to provide a continuous bore through each respective group of aligned solenoids, a plurality of bistable magnetic rods inserted in respective bores of the groups of aligned solenoids and serving as the bistable elements therefor, each bistable magnetic rod comprising a conductive substrate having a thin bistable magnetic film deposited thereon and a thin non-magnetic conductive overcoating deposited over said magnetic film, said conductive overcoating having a thickness less than 5,000 angstroms, and a helical solenoid wound on each rod over its respective conductive overcoating and electrically connected to the substrate at one end of its respective rod via its respective conductive overcoating.

11. The invention in accordance with claim 10, wherein said overcoating is gold.

12. The invention in accordance with claim 10, wherein the other end of each rod has a helical solenoid from another rod electrically connected to the substrate at said other end.

References Cited UNITED STATES PATENTS 3,411,892 11/1968 Yozo Saski et al. 29-l83.5 3,354,447 11/1967 Shintaro Oshima 340174.1 3,183,492 5/1965 Chow et a1. 340174 3,375,091 3/1968 Feldtkeller 29199 X 3,364,064 1/1968 Wijburg 29199 X 3,343,145 9/1967 Bertelsen 340-174 3,370,979 2/1968 Schmeckenbecher 117-2l7 JAMES W. MOFFITT, Primary Examiner US. Cl. X.R. 

